Measurements of enzyme activity serve as important tools for the diagnosis of disease processes and for basic research. We are seeking to develop new substrates for enzymes that improve the ability to measure or the specificity of measuring enzyme activity. We have found that linking small chromogenic and fluorogenic substrates to polymer molecules serves as an approach to prepare substrates with large molecular size. Initial results were described in Clinical Chemistry 2001;47:215-222. Continuing studies have examined further models analyzing effects of substrate size on enzyme activity. These studies have examined effects of substrate size on antibody inhibition of enzyme activity (J Clin Lab Anal 2001;15:64-70) and on the activity of enzymes that have an active site located within a tubular structure that serve as a size-dependent filter using proteasomes as a model (J Prot Chem 2002;21:333-337.) We have been working on better physical characterization of the size of the new synthetic substrates using light scattering techniques and development of substrates for other classes of proteases such as metalloproteases.